Looks like Microsoft decided that a number of Xbox 360 owners require some nudging in order to upgrade to the Xbox One, so they’re sending a promotion that gifts them with a $75 promotional code if they purchase an Xbox One or Xbox one Bundle.
Basically, those that received the promotion will be able to get a free game and a portion of another to start out their current-gen collection.
It’s worth mentioning that, while many are reporting having received the promotion, apparently not everyone did (I sure didn’t), and that whether you already have an Xbox One or not doesn’t really matter to the possibility of receiving it. In order to check out if you’re affected, all you have to do is to turn on your Xbox 360 and connect to Live.
You can check out two screenshots of the promotion just below, courtesy of NeoGAF user BeforeU.